A gasoline direct injection engine is an engine that has advantages of both a gasoline engine and a diesel engine. These advantages generate a high fuel economy engine. In the gasoline direct injection engine, fuel is directly injected into a combustion chamber. In this gasoline direct injection engine, intake flow is a very important factor in determining engine efficiency. The intake flow is determined by the shape of the intake ports, the combustion chamber, and the pistons. Research on intake flow characteristics has continuously been undertaken to prevent wall flow of the intake air and to make the intake air mix with the fuel in a short time period.
Various such engines are described in the art. For example, a gasoline direct injection engine is disclosed by an applicant of this application in a Korean patent application No. 10-2001-0059601. The gasoline direct injection engine disclosed in the above-stated application includes a combustion chamber that is defined by a lower surface of a cylinder head and an upper surface of a piston. The piston is disposed within a cylinder and an intake valve and an exhaust valve are respectively disposed within an intake port and an exhaust port. The intake and exhaust ports are formed on both sides of the cylinder head. A spark plug is disposed at a center portion of the lower surface of the cylinder head and a fuel injector, for injecting fuel into the combustion chamber is also disclosed.
The combustion chamber is formed between a bottom surface of the cylinder head including an intake-valve-side slanted surface and an exhaust-valve-side slanted surface. An upper surface of the piston has a pent roof shape including an intake-valve-side slanted upper surface and an exhaust-valve-side slanted upper surface corresponding, respectively, to the intake-valve-side slanted surface and the exhaust-valve-side slanted surface.
A bowl is formed in a pent-roof-shaped protrusion portion that is formed on the intake-valve-side slanted upper surface and the exhaust-valve-side slanted upper surface. A bottom surface of the bowl is formed such that the bowl has a depth increasing on approach to the intake side.
A split wall is disposed within the intake port such that the intake port is divided into an upper passage, acting as a passage for generating a tumble flow, and a lower passage. A valve, which operates to open or close the lower passage by a control of an engine control unit is disposed at an inlet of the lower passage.
The bowl looks like a trapezoid when it is seen from the top and a width of an exhaust-valve-side portion of the bowl is greater than a width of an intake-valve-side portion thereof. A bottom surface of the bowl is formed as a curved surface such that a depth of the bowl substantially gradually increases on approach to the intake-valve-side portion thereof. An intake-valve-side ramp of the bowl is substantially vertical. The spark plug is disposed to inject fuel from a portion near the intake port toward a combustion chamber above the bowl.
Therefore, if the piston moves down to introduce intake air, intake air passes through the upper passage and then enters the combustion chamber in a tumbled state. The tumble flow is strengthened in the combustion chamber. Then, the piston moves up to compress the air-fuel mixture and the tumble flow is further increased while the mixture is flowing in the bowl. While an air flow is introduced from the wide exhaust-valve-side portion of the bowl to the narrow and deep intake-valve-side portion of the bowl, a degree of a tumble flow is increased. The flow collides with the ramp and then the flow rapidly rises thereby reaching the spark plug.
During such a process, fuel injected from the fuel injector cooperates with the strong tumble flow, so that injected fuel does not collide with the bottom surface of the bowl of the piston and is evaporated in a short time. The air-fuel mixture then reaches the spark plug and is burned.
However, with such a conventional configuration, a protrusion is formed in an air inlet portion of the bowl. The protrusion causes a separation phenomenon that a portion of the tumble flow is separated instead of being sent toward the intake-valve-side portion of the bowl. Such separation phenomenon may deteriorate the combustion stability and the injected fuel may contact oil so that a carbon layer can be formed. Furthermore, a summit portion of the protrusion portion is formed as a flat portion. Therefore, a portion of the tumble flow may go beyond the side surface of the bowl and around the flat portion. This may decrease an integrity of the tumble flow and also weaken a squish flow, and this may be a cause of incomplete combustion. Still furthermore, valve pocket is divided by a step portion so that carbon may accumulate. The step portion may hinder a spread of firing and this may cause a knocking.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is known to a person skilled in the art.